(2) SCHWEIZ MINERAL PETROGR MITT. 79,. 79-87,1999. Pre-Alpine geochronology of the Central, Western and Southern Alps by Urs Schaltegger1 and Dieter Gebauer1. Abstract crucial role m unravellmg the complex and polyphase evolutionary history of the Alpine basement In this review, mostly recent data will be used to set up an orogenic timetable for the pre-Alpine evo¬ lution, starting with Archean Information hidden in zircon cores and fimshmg with widespread metasomatic alter¬ ations of Triassic/Liassic age, detected best in the Southalpine domain The evolution of the Alpine basement con¬ fined by these age limits comprises post-Panafncan clastic Sedimentation on the Gondwana shelf, the Cambrian Early Ordovician oceanic period, Devonian mafic and felsic magmatism, orogenic cycles in the Ordovician and Permo-Carboniferous and voluminous magmatism at different moments during this evolution. Geochronology plays. a. -. Keywords Alpine basement, geochronology, U-Pb, Sm-Nd, K-Ar,. Introduction Over three decades of isotopic dating in the Alpine basement have repeatedly shown pieces of evidence that this basement is polyphase and con¬ sists of recycled components as old as 3 43 Ga (Gebauer, 1993)! The aim of reconstructing such a complex evolution seems too difficult to reach, especially m the high-grade metamorphic terrains of the Central Alps where many ages are partly or completely obhterated Various new technologies offer, however, new means with highest temporal resolution that allow to decipher multistage evolutions to a high degree of precision even in the ar¬ eas of strongest Alpine overprint The pre-Alpme evolution has mamly been recorded by the U-Pb zircon and Sm-Nd whole-rock and mineral Sys¬ tems, which can be exploited by means of microanalytical approaches such as Single gram or msitu spot dating This review of pre-Alpme isotopic ages will sum up the present day knowledge of the geochronology of mamly crystalline parts of the Central Alps and presents a kind of geodynamic. time-table for its evolutionary history We will concentrate on key areas within the Central, Western and Southern Alps For a more compre-. Ar-Ar, Re-Os, Rb-Sr. hensive overview on the Pre-Alpine evolution of different tectonic units of the Central Alps, we also refer to review articles of Gebauer (1993) and. Schaltegger (1994) Proterozoic and Archean inheritance. DETRITAL ZIRCONS IN METASEDIMENTS The oldest age record comes from detrital zircons and zircon cores from metasedimentary gneisses Age determinations of different units revealed the existence of ancient components of 3 43, 2 8-2 6, 2 3-2 1, 1 0, 0 65-0 57 Ga age (Böhm, 1996, Gebauer, 1992,1993, Gebauer and Quadt, 1991, Schaltegger, 1993) In most metasedimen¬ tary rocks the inheritance turned out to be of. polyphase character. Conventional upper inter¬. cept ages are therefore considered as mixtures of different components, even in the case of abraded Single crystal analyses The distribution of ages suggests that the provenance of the detntus was one or more ancient craton(s) with Gondwana affinity Some detrital zircons occasionally yielded Grenville ages around 1 Ga (Gebauer, 1992, Böhm, 1996) Presence or absence of a 1 0 Ga oro-. Department of Earth Sciences, Swiss Federal Institute of Technology, ETH-Zentrum, CH-8092 Zürich, Switzer¬ land <schaltegger@erdw ethz ch> <gebauer@erdw ethz ch>.
(3) 80. U.. SCHALTEGGER AND. is a determining factor for speculathe provenance. The existence of Ga-old detrital zircons thus suggests that the Brasilian and/or the Guyana shield may be the provenance area for metasediments of the Alpine basement rather than the West African Craton. The 0.6 to 0.65 Ga age is found in Upper intercepts of many granitoid rocks of Ordovician to Tertiary age, suggesting that most of the source rocks show a predominant imprint of the Pan-African oroge¬ ny. The youngest detrital zircon components in metasediments of the Gotthard (Gebauer et al., 1988) and Aar massifs (Schaltegger, 1993), or the Southern Alps (Grünenfelder et al., 1984) are of Pan-African age, similar to metasediments of the European Variscides outside the Alps. They thus indicate post Pan-African deposition of the sedimentary protoliths. A recent river sand of the Po-delta, which samples large proportions of the Western-, Central- and Eastern Alps contains, as expected, only ca. 4% Alpine, i.e. almost exclusively Oligocene zircons (Gebauer, 1992). Detri¬ tal zircons from a Miocene marine molasse sand¬ stone, on the other hand, are mostly of Alpine age, the pre-Alpine grains being predominantly of Hercynian age, followed by Caledonian and Panafrican ages. Only few older grains at ca. 1.0 Ga, 2.1-2.3 Ga and 2.7 Ga were detected (Gebauer, 1992).. D.. GEBAUER MANTLE EVENTS. genic phase tions about. 1. Nd,Hf MODEL AGES. Another piece of information about provenance is offered by Nd and Hf model ages on a variety of mafic to acid rocks. Model ages of granites from the Alpine basement scatter around a mean value of 1.5-1.6 Ga. Only the Austroalpine Silvretta nappe seems to comprise older components in higher proportions, indicated by model ages rang¬ ing between 1.9 and 1.7 Ga (Poller et al., 1997a; Biino et al., 1994). Hf model ages of 1 Ga from highly evolved granites of the Aar massif (Schaltegger and Corfu, 1992) may be caused by addition of juvenile partial melts in late Variscan magmatic activity (ca. 300 Ma). Addition of subcontinental mantle-derived magmas to ca. 1.7 Ga-old crust was postulated to produce mod¬ el ages at around 0.9 to 1.3 Ga (Stille and Steiger, 1991). It is important to note that these model ages reflect mean ages of mantle differen¬ tiation ("mean crustal residence ages") and only accidentally date a geological event.. Mafic to ultramafic rocks often display Nd and Hf model ages of 0.9-1.0 Ga, interpreted as a memo¬ ry of a mantle event, which can be either partial melting and/or metasomatism in the subcontinen¬ tal lithosphere (Stille and Schaltegger, 1993). Beside Nd model ages, discrete mantle events may be recorded by zircons of mafic and ultramafic rocks that were formed during partial melting of mantle material and survived the subsequent high-T evolution within the mantle. Ages of 3.17, 2.67, 2.45, 1.72 and 1.27 Ga were reported from the Alpe Arami peridotite of the Cima-Lunga nappe (Gebauer et al., 1992a) and an eclogite from the Gotthard massif (Gebauer et al., 1988). Sm-Nd whole-rock reference lines from MOR-type amphibolites, banded amphibolites and ultramafic rocks of the Penninic domain (Schenk-Wenger and Stille, 1990; Stille and Tatsumoto, 1985) have been interpreted to date their emplacement 1.0 Ga ago. This interpretation is challenged by U-Pb age determinations on zir¬ cons of the Cima di Gagnone eclogites (Gebauer, 1994), which yielded a much younger age of 528 ± 6 Ma, interpreted as emplacement as well. Late Precambrian mafic to ultramafic igneous rocks The polyphase Alpine metamorphic basement hosts large volumes of mafic rocks, today occur¬ ring as amphibolites of mid-ocean ridge to islandarc composition. They are part of ancient conti¬. nental margin or wedge sequences including metasediments of all kind plus obducted rem¬ nants of oceanic crust (Gotthard: Biino, 1995; Sil¬ vretta: Biino et al., 1994). Direct dating of the em¬ placement of these rocks is precluded by their lack of primary zircon, with a few exceptions: Gebauer et al. (1988) reported a SHRIMP U-Pb age of 870 Ma from eclogite zircons of the Gotthard massif and interpreted it as dating the intrusion of the protolith. A diorite of the Sil¬ vretta nappe with an age of 609 ± 3 Ma (Schalt¬ egger et al., 1997) post-dates the formation of the volcano-sedimentary sequences in which it in¬. truded. Precambrian-Early Cambrian Sedimentation The metasedimentary sequence of the Southern Alps, the Austroalpine nappes and the External Massifs contain a mixed association of clastic Sed¬ iments and meta-carbonates, interlayered with.
(4) PRE-ALPINE GEOCHRONOLOGY. amphibolites of igneous and sedimentary origin. Their Sedimentation age is constrained by the youngest detrital zircons, which have been dated between 0.62 and 0.55 Ga (Schaltegger, 1993; Böhm, 1996; Gebauer and von Quadt, 1991;. Grunenfelder et al., 1984). These sedimentary basins have been interpreted as continental mar¬ gin deposits during the break-up of the Gond¬ wana continental margin ca. 0.6 Ga ago into sev¬ eral microcontinents or continental fragments, which have been re-assembled during the Variscan collision. The resulting sediment series became part of the accretionary wedge in the Or¬ dovician orogenic cycle in the Alpine realm, dur¬ ing which they were overprinted in eclogite and/or granulite facies (see below). Cambrian magmatism The Cambrian time period is characterized by the existence of several ocean basins that developed. between different microcontinents, Gondwana and Laurasia. Remnants of oceanic sequences can found spread over the whole Alpine ränge. Metagabbros and metabasalts display very differ¬ ent geochemical characteristics, which allow the interpretation that constrasting types of oceanic and active margin settings must have been pre¬ served in the Penninic area (Cima di Gagnone, 528 ± 6 Ma, Gebauer, 1994; Biasca, 521 ± 8 Ma, Gebauer, 1994) and in the Silvretta nappe (Old¬ er Orthogneisses, 525 ± 5 Ma; Schaltegger et al., 1997, Muller et al., 1995). An ophiolitic sequence is known from the western Alps (Chamrousse, 496 ± 6 Ma; Menot et al., 1988). Coeval granitoid be. rocks are interpreted as plagiogranites (Silvretta nappe, 532 ± 30 Ma; Muller et al., 1996) or alka¬ li granites, e.g. the 500 +3/-4 Ma old Thyon meta¬ granite (Siviez-Mischabel; Bussy et al., 1996). A granitoid pluton intruded into dacites of the Southern Vanoise (western Alps) yielded an age of 507 ± 9 Ma (Guillot et al., 1991). The anatec¬ tic Mönchalp granite in the Silvretta nappe yield¬ ed conflicting data between 527 and 460 Ma, indi¬ cating a complex evolution in a polyphase conti¬ nental basement (Poller, 1997; Poller et al.,. 1997b).. Ordovician magmatism and metamorphism A complete orogenic cycle of Ordovician age has been discovered and dated by means of U-Pb on zircon in the Helvetic, Penninic, Austroalpine and Southern Alpine domains. Metagabbros from the Silvretta nappe were dated at 467 ± 14 Ma. 81. (Poller, 1997), from the Gotthard massif at 467 +5/-4 Ma, from the Tavetsch unit at 471 +6/-7 Ma (both in Oberli et al., 1994), and from the Aar massif at 479 ± 5 Ma (Abrecht et al., 1995).These ages were interpreted as intrusion ages; since the rocks were overprinted by HP-metamorphism subsequent to intrusion, the ages might be slight¬ ly biased towards younger lead loss at ca. 468 Ma. 1990; Gebauer and von Quadt, also indicated by a Sm-Nd garnet Ma (Gebauer et al., 1988). The Oberstaffelgneis in the Gotthard massif has an 478 of +8/-4 Ma and is thus coeval with the age gabbros (Sergeev and Steiger, 1995). The same 479 ± 6 Ma has been reported from the age. (Gebauer,. 1991). This age of 461. is. ± 25. Brianconnais domain (Vanoise; Bertrand and Leterrier, 1997).The "Younger Orthogneisses" of the Austroalpine Silvretta nappe present the largest magmatic area of Ordovician age. They in¬ clude mainly granitoid rocks (Flüela granite asso¬ ciation) and intruded within a time ränge of 460 to 420 Ma (Poller et al., 1997 a, b). A metamorphic evolution of Ordovician age has been reported from the Aar massif, including U-Pb age data from zircons in metapsammites at 450 ± 5 Ma and a subsequent anatectic stage rep¬ resented by migmatites, dated at 456 ± 2 and ca. 445 Ma (Schaltegger, 1993). The anatectic Streifen gneiss (Gotthard massif) fits into the same scenario, with an age of 445 +4/-5 Ma (Sergeev and Steiger, 1995), as well as augengneisses from the Mont Blanc massif at 453 ± 3 Ma (Bussy and von Raumer, 1993). In the South Alpine Strona Ceneri Zone acid Ordovician mag¬ matism (Boriani et al., 1981) with ages between Ma and 480 Ma (Koppel and Grunen¬ 1971; Hunziker and Zingg, 1980) is a common phenomenon; the existence of a meta¬ morphic overprint of Ordovician age is, however, quite controversely debated (e.g. recently by Boriani and Villa, 1997; Zurbriggen et al., 1997; Romer and Franz, 1998).. ca 450. felder,. Devonian - Lower Carboniferous magmatism. Devonian oceanic basalts and gabbros have been found in different portions of the Variscan orogen, e.g. in monometamorphic series of the western External Massifs (Menot et al, 1987). Nothing equivalent is known from the Central Alps. Alpine HP metagabbros and one eclogite from the Western Alpine Sesia-Lanzo Zone yielded U-Pb zircon ages around 355 Ma for the intrusion of the MORB-type protoliths (Rubatto and Gebauer, 1997). Calcalkaline volcanism at 355 ± 6 Ma has also been inferred from granulite-facies.
(5) 82. U.. SCHALTEGGER AND. metagranitoids of the Ivrea Zone (Vavra et al., 1996). This implies that at least parts of the Ivrea Zone were in a supracrustal position around the. Devonian-Carboniferous boundary. Variscan metamorphism The age of the Variscan metamorphic overprint in the Alpine basement is not known at a high level of confidence. The degree of Variscan metamor¬ phism seems to be variable among central Alpine basement units: Aar and Gotthard massifs only record lower to middle amphibolite facies para¬ geneses, whereas in the other areas Upper amphi¬ bolite-facies conditions leading to anatexis were. reached. High-temperature metamorphism was dated at 330 ± 2 Ma (Bussy et al., 1996) in. metasedimentary sequences of the middle Pen¬ ninic area. Migmatites from the Aiguilles Rouges and Mont Blanc massifs show ages at 317 ± 2 and 321 Ma (Bussy and von Raumer, 1993; Bussy and Hernandez, 1997). In the lower-grade Aar and Gotthard massifs, zircons do not define Variscan but Ordovician U-Pb ages in many cas¬ es. Exceptions are zircons in retrograded Ordovi¬ cian eclogites, which formed 330 ± 3 Ma ago by de¬ composition of mainly omphacite; this age agrees with a rutile U-Pb age of 329 ± 2 Ma from the same area (Susten; both ages in Schaltegger, 1993). Variscan metamorphism seems to be coeval to or even younger than the magmatism; this fact might be indicative for an elevated heat flow due to the thermal impact of large masses of granitoid melts intruding into the crust, creating a regional. contact-metamorphic overprint. The Variscan metamorphic event in the Sil¬ vretta nappe seems to be slightly younger, indi¬ cated by U-Pb ages around 306 ± 2 Ma on staurolites (Frei et al., 1995), identical to retrograde zircon crystallization in a former eclogite in the same area at 301 ± 3 Ma (Liebetrau et al., 1996).. Boriani and Villa (1997) deduced an age of ca. 340 Ma from complex Ar-Ar mineral patterns for Variscan amphibolite-facies metamorphism in the Southalpine Strona-Ceneri Zone. Variscan to Permian late-orogenic magmatism Variscan magmatism of the Central Alps is de¬ scribed in detail in the review articles of Bonin et al. (1993) and Schaltegger (1994). Therefore, mainly recently published data will be treated in the next paragraph. The late Variscan magmatic activity can be separated into several distinct episodes:. D.. GEBAUER. Visean magmatism: High-K diorites, syenites and granites occurring in the Aar massif (Tödi, Giuv-Punteglias) were dated at 334 ± 2 Ma (Schaltegger and Corfu, 1992,1995). A recent age determination of the Pormenaz monzonite in the Aiguilles-Rouges massif yielded an identical age of 333 ± 2 Ma (Bussy et al., 1998). A calc-al¬ kaline granitoid suite in the Bernina nappe in¬ truded between 338 and 330 Ma (von Quadt et al., 1994). Westphalian-Stephanian magmatism: Calc-al¬ kaline granitoids and diorites of the Aar massif in¬ truded between 310 ± 3 and 308 ± 2 Ma (Schalt¬ egger and Corfu, 1992). Different gabbroic to granitoid rocks from the Aiguilles-Rouges (Val¬ lorcine and Montenvers granites; gabbroic, dioritic and granitoid portions of the Fully intrusive complex), as well as a rhyolite at the contact of the Mont Blanc granite are precisely dated at an age of 307 ± 1 Ma (Bussy and Hernandez, 1997); the Mont Blanc granite itself is dated at 304 ± 3 Ma (Bussy and von Raumer, 1993). The volcanosedimentary basin of Salvan-Dorenaz was de¬ posited within an time frame from 307 to 297 Ma (Capuzzo and Bussy, 1998). The subalkaline Central Aar granite and Gamsboden and Fibbia gneisses from the Gotthard massifs have intrusion ages of 298-300 (Schaltegger and Corfu, 1992; Sergeev et al., 1995), a slightly younger pulse at ca. 295 Ma is found in the Gotthard massif only (Sergeev et al., 1995).The orthogneisses contain¬ ing the UHP-peridotites at Alpe Arami (Cima Lunga-Adula nappe) are also derived from 296 Ma old granites (Gebauer, 1996). A similar age ränge (288 ± 7 to 295 ± 12 Ma) is given by von Quadt et al. (1994) for granites of the Aus¬ troalpine Bernina nappe. Generation and emplacement of these granit¬ oid melts are suggested to be short-term pulses that are controlled by basin-and-range-type ex¬ tensional tectonic processes, thermal relaxation and erosion of the lithospheric mantle, uplift and decompressional melting (Schaltegger, 1997). Permian to Triassic magmatism: Granitoid rocks in Upper crustal Penninic units ranging from calc-alkaline to alkaline compositions have intru¬ sion ages scattering between ca 270 and 280 Ma: Truzzo and Roffna granitoids in the Tambo and Suretta nappes (268 ± 2 Ma; Marquer et al., 1998), Grand-Laget and Randa (Siviez-Mischa¬ bel; 269-275 Ma; Bussy et al., 1996) and Dora Maira granitoids (300-265 Ma; Bussy and Cadoppi, 1996; Gebauer et al., 1997). This age ränge overlaps with gabbro intrusions at the base of the crust: e.g. the Fedoz/Braccia gabbro at 270 +6A-4 Ma (Hermann et al., 1997)..
(6) 83. PRE-ALPINE GEOCHRONOLOGY Similar lower crustal intrusions are typical for Alpine basement: Main Gabbro-Diorite Body yielded ages around 280 Ma for both Main Gabbro (Gebauer, 1993) and Diorite bodies (Pin and Sills, 1986). The zircon data from further mafic rocks of the Ivrea Zone (Gebauer et al., 1992b; Gebauer, 1993) have not substantiated interpretations of Sm-Nd whole rock data (Pin and Sills, 1986; Voshage et al., 1987; Lu et al., 1997) that part of the Mafic Formation in the Ivrea Zone is of Early Paleozoic age. Many felsic to intermediate mag¬ matic rocks of the Southern Alpine domain out¬ side the Ivrea Zone yielded ages in a wide ränge between 262 Ma and 295 Ma (Barth et al., 1994;. the Ivrea Zone and Southern. The. Boriani et al., 1981; Hunziker and Zingg, 1980; Koppel, 1974; Stille and Buletti, 1987). As the ränge of initial Sr-isotopic compositions is similar to those of the Main Gabbro-Diorite Body of the Ivrea Zone (Pin and Sills, 1986; Voshage et al., 1987), a genetic relation between these rocks is possible. However, a derivation of the felsic mag¬ matic rocks by degranitization of metasediments of the Ivrea Zone can be excluded with the same arguments from In. the. Sr isotope geochemistry. Austroalpine-derived Sesia-Lanzo. Zone predominantly bimodal magmatism is of Permo-Carboniferous age. This applies for a num¬ ber of the granitic and gabbroic protoliths of Alpine HP rocks in the Monte Mucrone area and the Aosta Valley (Bussy et al, 1998; Rubatto and Gebauer, 1997). The Permian magmatism is geochemically bi¬ modal: Mafic intrusions at the base of the crust seem to trigger acid volcanism and plutonism in the upper crust in an overall divergent, strike-slip dominated setting. The geodynamic scenario com¬ prises large shear zones running through the Variscan orogen, reaching deep into the litho¬ sphere and causing lithospheric extension along. narrow corridors. Triassic to Jurassic metasomatism, metamorphism and magmatism. Determination of the age of Variscan regional metamorphism in the Ivrea Zone is rendered dif¬ ficult by a series of thermal and/or hydrothermal pulses probably related to multi-episodic under¬ plating after 270 Ma. As a consequence, data for the peak of metamorphism ränge from ca 296 Ma 273 Ma (Henk et al., 1997; Koppel, 1974; Vavra and Schaltegger, 1999; Vavra et al., 1996, 1997). Local thermal pulses, at least partly accompanied by fluid infiltration are documented by zircon and monazite at 261 ± 4 Ma, 226 ± 5 Ma. to. and 210 Valleys. ±. 10. Fiorina Vavra and. Ma in the Mastallone and. (Vavra et al, Schaltegger, 1999).. 1996,. 1997;. A magmatic-metamorphic episode around Ma has been derived from different litho¬ from the Ivrea and Strona-Ceneri zones, e.g. from muscovites of pegmatites of the Ivrea Zone (Rb-Sr; 252 ± 10 Ma; Hunziker, 1974) and the Lake Massif (259 ± 8 Ma; Boriani et al., 1981), which somewhat disagree with ages for Southern Alpine pegmatites between 200 and 230 Ma (Sanders et al., 1996). Sm-Nd mineral ages by Voshage et al. (1987) gave ages between about 230 Ma and 270 Ma for granulite facies rocks of the Ivrea zone, arguing that at least local¬ ly amphibolite facies conditions prevailed during the Triassic between Val Sesia and Val Strona. Similarly, metamorphic zircon domains of a py¬ roxenite within the Lower Layered Group close to the Balmuccia peridotite also argue for local high-grade metamorphic conditions, in this case at 265 +4/-5 Ma (Gebauer et al., 1992b). In the NE part of the Ivrea Zone close to Ronco, metasedi¬ ments contain a boudinaged amphibolite with a preserved gabbroic core that yielded an age of 238 ± 1 Ma, interpreted as protolith age (Gebauer, 1993). Based on this value, local deformation un¬ der amphibolite facies conditions would have to 250. logic units. be. 238. considered contemporaneous or younger than. Ma.. The 250 Ma age is common value for acid vol¬ canic rocks and tuffs interlayered in carbonates that were deposited at the Anisian/Ladinian boundary in the southern Alps (Mundil et al., 1996;. Brack. An. et al.,. important. 1996). thermal. and. hydrothermal. event at around 210 Ma has repeatedly been re¬. ported by zircon and monazites from different parts of the Ivrea zone, causing the formation of metasomatic mineral assemblages. The same age is recorded by Re-Os and Rb-Sr ages from alka¬ line ultramafic pipes crusscutting gabbros (Biino and Meisel, 1996). Gabbros from around the ul¬ tramafic Finero complex were dated at between 230 and 210 Ma by Sm-Nd mineral isochrons and Pb-Pb zircon ages (Lu et al., 1997); zircon from stratiform chromitite layers was dated at 207 ± 5 Ma (von Quadt et al., 1993). The intrusion of a syenite pegmatite within the Finero peridotite yielded a Pb/Pb zircon evaporation age of 225 ±13 Ma (Stähle et al., 1990). Finally, mica ages ränge from 230 Ma to 180 Ma (McDowell and Schmid, 1968; Hunziker, 1974) indicating that cooling be¬ low the individual blocking temperatures oc¬ curred at different times in different parts of the Ivrea Zone. The youngest pre-Alpine intrusions in the Ivrea zone are Middle Jurassic old gabbros.
(7) 84. U. SCHALTEGGERAND. (177 Ma) of the Sesia valley (Gebauer, 1993) The effects causing this ränge of apparent ages around Ma are assigned to pervasive fluid flow through the crust during multi-episodic under¬ plating of mainly mafic rocks mto the lower crust and by incipient continental nftmg at the onset of the opening of the Neo-Tethys ocean, coeval to voluminous Sedimentation in the Rhetian of the 210. Southern Alps. Similar effects are also registered in middle and upper crustal levels of many areas of the Cen¬ tral, Western and Southern Alps Rb-Sr wholerock ages of 230 ± 8 Ma (Schaltegger, 1990) and 185 ± 17 Ma (Knill, 1996) were reported for gran¬ itoids from Helvetic and Penninic domains, re¬. spectively For the internal Dora Maira Massif of the Western Alps, SHRIMP-data indicated that nu¬ merous zircon domains were partly or completely reset in the time ränge 260-210 Ma (Gebauer et al, 1997), linked to a pervasive Mg-metasomatism, which developed along former shear zones within a 275 Ma-old granite The resulting Mg-nch rocks, Mg-chlorite-quartz-muscovite schists are comparable to the leucophylhtes" of the Eastern Alps and very probably represent the secondstage protoliths to the Alpine UHP pyrope quartzites (Gebauer et al, 1997) ". Conclusions A wealth of new geochronological Information has been acquired since our last reviews from the Alpine basement (Gebauer, 1993) and the exter¬ nal massifs of the Central Alps (Schaltegger, 1994) Recent data shed new light onto two very interesting and emgmatic time penods (l) the. Cambrian to Ordovician orogenic cycle, and (n) the Tnassic-Liassic metasomatic and/or thermal overprint of the Southern Alpine basement The presence of an Ordovician orogenic cycle going through subduction, eclogite- and subse¬ quent granuhte-facies metamorphic overprinting and finally decompressional melting of the crust between ca 480 and 440 Ma has one important implication the pre-Alpme dominant metamor¬ phic overprint is clearly of Ordovician age m ar¬ eas, where Variscan metamorphic conditions re¬ mained in amphibolite facies The recent debate about the age of metamorphism in the Southern Alpine basement (Boriani and Villa, 1997, Zurbriggen et al, 1997, Romer and Franz, 1998) origmates from this problem The Upper Triassic thermal and/or hydrothermal overprint of the Southern Alpine crust at ca 220 to 200 Ma has even more spectacular imphcations monazite. D. GEBAUER. U-Pb and other mineral ages (Ar-Ar, Rb-Sr) are very likely biased by this event and only carry in¬ complete and disturbed Information about re¬ gional metamorphism Former ideas about meta¬ morphic zoning, uplift histories and crustal tec¬ tonics derived mamly from the Ivrea and StronaCeneri zones become strongly questionable m this light and the evolutionary history may be re¬ solved into a series of distinct thermal/hydrothermal events between 300 and ca 180 Ma. Acknowledgements Comments by J M Bertrand, F Bussy, H W Hansmann and J von Raumer were highly appreciated. References. Abrecht,. Biino, G G and Schaltegger, U (1995) Building the European continent Late Proterozoic - Early Paleozoic accretion in the Central Alps of Switzerland EUG 8, Strasbourg Terra abstracts 7/1 105 Barth, S, Oberli, F and Meier, M (1994) Th-Pb ver J,. U-Pb isotope systematics in allanite from co-ge rhyolite and granodiorite imphcations for geochronology Earth Planet Sei Lett, 124,149-159 and Leterrier, J (1997) Gramtoides d äge Paleozoique inferieur dans le socle de Vanoise meridionale geochronologie U-Pb du metagranite del Arpont (Alpes de Savoie, France) C R Acad Sei Paris, Sei Terre Planet 325,839-844 Biino, G G (1995) Pre-Variscan evolution of the eclogitised mafic rocks from the Helvetic basement of the central Alps Eur J Mineral 7,57-70 Biino, G G and Meisel, Th (1996) Ar-Ar, Re-Os, Rb-Sr, Sm-Nd and U-Pb isotopic, trace element and petrologie study of alkaline minerahzed ultra¬ mafic pipes in the Ivrea Verbano zone (Italy) Abstract, Schweiz Mineral Petrogr Mitt 76,98-99 Biino, G G, Meisel, Th Nagler, Th and Kramers, J (1994) Whole-rock chemistry and isotope chemistry of metasediments in the Silvretta nappe and the ear ly crustal history of the Alpine basement Abstract, OSchweiz Mineral Petrogr Mitt 75,293-294 Böhm, Ch O (1996) Provenance and pre-Mesozoic evolution of the Lucomagno unit (Central Swiss Alps) A geochemical, isotopic and geochronologi¬ cal approach Unpubl Ph D Thesis, ETH No 11773, 144 pp Bonin, B (coord) Brandlein, P, Bussy, F, Desmons, J, Eggenberger, U, Finger, F, Graf, K Marro, sus. netic. 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(8) 85. PRE-ALPINE GEOCHRONOLOGY the Serie dei Laghi orthogneisses - Ticino, Switzerland) Rend Soc. (Northern Italy Ital. Mineral. Petrol 38,191-205. and. Brack, P, Mundil, R, Oberli, F, Meier, Rieber, H (1996) Biostratigraphic and radiometric age data question the Milankovitch charactenstics M. of the Latemar cycles (Southern Alps, Italy) Geol¬ ogy 24/4,371-375 Bussy, F and Cadoppi (1996) U-Pb zircon dating of granitoids from the Dora-Maira massif (western. Italian Alps) 217-233. Mineral Petrogr. Schweiz. Mitt 76,. Bussy, F and Hernandez, J (1997) Short-lived bimodal magmatism at 307 Ma in the Mont-. Blanc/Aiguilles-Rouges area a eombination of de¬ compression melting, basaltic underplating and crustal fractunng Abstract 3rd Workshop on Alpine Geological Studies, Oropa-Biella, Quad Geodin Alpina. e. Quaternana,. 4,. 2. Bussy, Fand von Raumer, J (1993) U-Pb dating of Pa¬ leozoic events in the Mont-Blanc crystalline massif, Western Alps Terra abstracts 5, Supplement 1. 382-383 Bussy, F, Delitroz, D, Fellay, R and Hernandez, J The Pormenaz monzonite (Aiguilles(1998) Rouges, Western Alps) An additional evidence for a 330 Ma-old magnesio-potassic magmatic suite in the Variscan Alps Abstract, Schweiz Mineral Petrogr. Mitt 78,193-194. Derron, M H Jaquod, J, Sartori, M and Thelin, Ph (1996) The 500 Ma-old Thyon meta¬ granite a new A-type granite occurrence in the west¬ ern Penninic Alps (Wallis, Switzerland) Eur J Min¬. 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Geochemical and Sm-Nd charactenstics a polymetamorphic S-type granitoid The Monchalpgneiss-Silvretta nappe/Switzerland Eur J Miner¬ al 9,411-422 Poller, U, Liebetrau, V and Todt, W (1997b) U-Pb single-zircon dating under cathodoluminescence control (CLC-method) application to polymeta¬ morphic orthogneisses Chem Geol 139,287-297 Romer, R L and Franz, L (1998) Ordovician Barrowtype metamorphism in the Strona-Ceneri Zone G (1997a). of. D. GEBAUER. (Northern Italy) dated by U-Pb on staurolite Schweiz Mineral Petrogr Mitt 78,383-395 D and Gebauer, D (1997) The Bonze Unit (Sesia-Lanzo Zone, Western Alps) a gabbroic Early Carboniferous intrusion that recorded the Hercy¬ nian and the Alpine metamorphism 3rd Workshop on Alpine Geological Studies, Biella-Oropa, Italy Sanders, C A E Bertotti, G, Tommasini, S, Davies, G R and Wijbrans, J R (1996) Triassic pegmatites in the Mesozoic middle crust of the Southern Alps Fluid inclusions, radiometric dating and tectonic im¬ phcations Eclogae geol Helv 98,505-525 Schaltegger, U (1990) Post-magmatic resetting of Rb-Sr whole rock ages - a study in the Central Aar Granite (Central Alps, Switzerland) Geol Rund¬ schau 79,709-724 Schaltegger, U (1993) The evolution of the poly¬ metamorphic basement in the Central Alps unravelled by precise U-Pb zircon dating Contrib Miner¬. Rubatto,. 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(10) PRE-ALPINE GEOCHRONOLOGY Isotopic Code Geophysical Monograph Series vol 95,269-276, Am Geophys Union, Washington P and Steiger, H R (1991) Hf isotope system atics in granitoids from the Central and Southern Alps Contrib Mineral Petrol 107,273-278. Stille, Stille,. P and Tatsumoto, M (1985) Precambrian tholentic-dacitic rock-suites and Cambrian ultra¬. mafic rocks in the Pennine nappe Systems of the Alps Evidence from Sm-Nd isotopes and rare earth elements Contrib Mineral Petrol 89,184-192 Vavra, G, Gebauer, D, Schmid, R and Compston, W (1996) Multiple zircon growth and recrystallization during polyphase Late Carboniferous to Triassic metamorphism in granulites of the Ivrea zone (Southern Alps) an ion microprobe (Shrimp) study. Contrib Mineral Petrol 122,337-358 Vavra, G, Schaltegger, U, Schmid, R Quadt, A (1997) Correlated rejuvenation. and von of zircon and monazite during post-granuhte facies fluid ac¬ tivity in the Ivrea Zone (Southern Alps) EUG 9,. Strasbourg, Terra Nova. Vavra,. G. facies. and. 9,. 87. in the Ivrea Zone (Southern Alps) Contrib Miner¬ al Petrol, in press von Quadt, A Ferrario, A Diella, V, Hansmann, G, Vavra, G and Koppel, V (1993) U-Pb ages of zircons from chromitites of the phlogopite peri¬ dotite of Finero, Ivrea zone, N-Italy Schweiz Mine¬ ral Petrogr Mitt 73,137-138 von Quadt, A, Grunenfelder, M and Buchi, Hj (1994) U-Pb zircon ages from igneous rocks of the Bernina nappe system (Grisons, Switzerland) Schweiz Mineral Petrogr Mitt 74,373-382 Voshage, H Hunziker, J C, Hofmann, A W and Zingg, A (1987) A Nd and Sr isotopic study of the Ivrea zone, Southern Alps, N-Italy Contrib Mineral Petrol 97,31-42 Zurbriggen, R Franz, L and Handy, M R (1997) Pre-Variscan deformation, metamorphism and mag¬ matism in the Strona-Ceneri zone (southern Alps of northern Italy and southern Switzerland) Schweiz Mineral Petrogr Mitt 77,361-380. Abstract Suppl 1,488 U (1999) Post granulite. Schaltegger,. monazite growth and rejuvenation during Permian to Lower Jurassic thermal and fluid events. Manuscript received April 17,1998, minor revision aeeepted August 27,1998.